In recent years, the advances in Forward-Error-Correction (FEC) and Bandwidth-Efficient Modulation (BEM) techniques have improved system capacity and service quality of various telecommunications significantly. For certain type of communications that are point-to-point and interactive, Variable Coding and Modulation (VCM) and Adaptive Coding and Modulation (ACM) techniques have the potential to further increase the system capacity as compared to a Constant Coding and Modulation (CCM) scheme. According to the message types and the link condition, a VCM or ACM transmitter utilizes FEC codes of various rates, sizes, utilizes modulations of different orders, and may include pilots to communicate to users with maximum efficiency. High rate codes and high order modulations are used to take advantage of favorable channel conditions while low rate codes and low order modulations are used to guarantee error free transmission when the channel conditions degrade.
Symbol timing, as well as carrier frequency and phase recovery for a VCM or ACM, presents several unique challenges. First, the application of near-Shannon-limit FEC codes demands the system to operate at lower and lower SNR levels. Second, the modulation scheme and the presence or absence of pilot symbols may be changed on a per-frame basis, which results in variable-length frames that preclude the application of conventional continuous phase recovery methods.
Novel receiver systems and methods are disclosed herein. Header information is used to facilitate coarse frequency and frame recovery. The coarse frequency and frame recovery is thereafter utilized to perform adaptive phase and frequency synchronization on a frame-by-frame basis.